A belt tensioner of this type is known from DE 297 10 011. The piston is shiftable in the housing from an initial position along an operating stroke under the effect of compressed gas generated by a gas generator after its activation. This movement of the piston is transferred to a belt reel by a toothing mounted at the piston and a pinion engaging therein, which is connected to the belt reel. The rotation of the belt reel serves to remove the so-called belt slack from the safety belt system. In the belt tensioner known, the exit port is arranged in the bottom of the piston. Its purpose is to reduce the pressure existing in the housing of the belt tensioner, this pressure existing after the piston has travelled its operating stroke. A pressure possibly existing would counteract a return movement of the piston. This return movement of the piston, however, is desirable and necessary when after the belt tensioning a reverse rotation of the belt reel is carried out, in which safety belt is released for the purpose of the force limitation.
It is disadvantageous in the case of the belt tensioner known that also during the operating stroke a certain portion of the compressed gas escapes through the exit port. This loss reduces the tensioning efficiency.
It is the object of the invention to further develop a belt tensioner of the aforementioned kind to the effect that a higher tensioning efficiency is achieved.